[cxx-interop] Support ownership annotations on FRT constructors
We were previously marking synthesized FRT initializers as
"returns_retained" unconditionally (for non-immortal types), which can
cause memory errors and also suppresses diagnostics about the lack of
such annotations.
This patch fixes that behavior, and also adjusts the source locations of
the synthesized C++ factory method to point to the actual constructor
decl when its location is available, so that the diagnostics make sense
when warning about the lack of annotations.
rdar://163127315
